Heating element having a high heating current

ABSTRACT

A heating element having a high heating current. The element has a wire resistor surrounded by insulating material consisting of a mixture of a pulverulent oxide and boron nitride.

United States Patent 191 Baldy et a1.

- Sept. 25, 1973 HEATING ELEMENT HAVING A HIGH HEATING CURRENT Inventors: Jean Baldy, Saint Cyr LEcole;

Patrick Coville, Le Val DAlbian, both of France U.S. Philips Corporation, New York, N.Y.

Filed: July 27, 1971 Appl. No.: 166,548

Assignee:

U.S. Cl. 338/238, 219/553 Int. Cl H01c 1/02 Field of Search 338/238; 219/544,

.219/553; 252/632; 174/118, DIG. 5

References Cited Primary Examiner-E. A. Goldberg Attorney-F rank R. Trifari 5 7 ABSTRACT A heating element having a high heating current. The element has a wire resistor surrounded by insulating material consisting of a mixture of a pulverulent oxide and boron nitride. 1

2 Claims, 2 Drawing Figures Patented Sept. 25, 1973 3,761,859

V I I ALZOQ Fig.2

INVENTORS JEAN BALDY PATRICK COVILLE BY 2 e 44 AGENT HEATING ELEMENT HAVING A HIGH HEATING CURRENT The present invention relates to a heating element having a very high thermal surface load, comprising at least one electric resistance heating wire asa core and an outer envelope which is insulated from the core by a compressed pulverulent material having a satisfactory thermal conductivity. In the commonly used elements, magnesium oxide or aluminium oxide is generally used as an insulating material. These materials have satisfactory thermal properties and may be subjected to the mechanical treatments which serve to compact the insulating material and to reduce its diameter.

However, for certain uses elements are desired which allow of a very high thermal surface load. Elements which comprise magnesium oxide or aluminium oxide as an insulating material are found to be not very suitable for this purpose, both due to the thermal conductivity and the electrical insulation. 7

An object of the present invention is to provide a heating element of the type described above, which is suitable for very high thermal loads. According to the invention the heating element is characterized in that the insulating material consists of an insulating mixture of a pulverulent metal oxide and boron nitride comprising more than percent by weightof boron nitride.

Boron nitride has an eminent thermal conductivity. However, it cannot be used as an insulating material in heating elements of the kind described above, because due to the lamella structure the powder flows away laterally during the mechanical treatment for the purpose of compacting the powder and reducing the diameter. Drawing becomes very difficult thereby. However, itwas found that mixtures ,of boron nitride with other insulating materials can be used in the manufacture of heating elements of the type. described, provided that the quantity of boron nitride lies above a certain indicated limit.

According to a preferred embodiment of the heating element according to the invention, the insulating material consists of a mixture which comprises between 5 and 30 percentby weight ofboron nitride, with the remainder magnesium oxide or aluminium oxide.

Preferably the mixture contains approximately 20 percent by weight of boron nitride when using magnesium oxide. When using'aluminium oxide, the mixture preferably containsmore than 10 percent by weight of boron nitride. I t a In US. Pat. No. 3,477,058 it is proposed to add up to 5 percent by weight of a material having a lamella structure such as boronnitride to-the magnesium oxide which is used as an insulating material in the manufacture of heating elements. Due to this-addition a greater magnesium oxide density iszobtained when using the conventional mechanical treatments.v Under these 'circumstances the electrical and thermalproperties of the elements are found to be enhanced. This enhancement is, however, ascribed to the higher density to be achieved with magnesium oxide.

The heating element described insaid patent from that according to the invention, in that in the heating element of the. invention essentially relies upon the thermal and electrically insulating properties of boron nitride.

differs In order that the invention may be readily carried into effect, it will now be further described in detail by way of example, with reference to the accompanying diagrammatic drawing, in which FIG. 1 shows a longitudinal section of a heating element and FIG. 2 shows a diagram of curves indicating the thermal gradient as a function of the quantity of boron nitride in mixtures containing aluminium oxide on the one hand and magnesium oxide on the other hand.

The heating element shown in FIG. 1 comprises an electrical resistance heating wire I of, for example, a Ni-Cr alloy which is coaxially located within an envelope 2 of, for example, stainless steel. The free space in between is filled with a compressed pulverulent insulating material 3. According to the invention the insulating material consists of a mixture which comprises boron nitride in a quantity of more than 5 percent by weight and furthermore a metal oxide such as aluminium oxide or magnesium oxide.

For reasons of practical considerations, measurements were performed on heating elements having an envelope 2 of nickel and a resistance heating wire 1 a AT/P in C/W/cm is a measure of the quality of the revelant heating element 1.

Tables I and II show values ofa for different compositions of the insulating material, the dissipated power was between 60 and W/cm and the temperature of the resistance wire 1 was between 200 and 300 C.

TABLE I pure NB NB NB Nli NB 0. w cm 3.8. as 3.5 3.3 2.0 1.0

' TABLE 11 pure- NB NB NB NB NB c./ /cm. a 2.4 2.2 1.9 2 2 The percentages mentioned in the Tables are percentages by weight.

These results were transferred to the diagram of FIG.

For heating elements having an insulating material consisting'ofa mixture of magnesium oxide and boron nitride an improvement of the thermal conductivity is observed in a ratio which is larger than 2 for a quantity of boron nitride of approximately 20 percent by weight. I

For heating elements having an insulating material consisting of a mixture of aluminium oxide and boron nitride the results obtained are still better: as from 'a quantity of approximately 10 percent by weight of boron nitride the improvement of the thermal conductivity is quite considerable and reaches a ratio of 3 at approximately 20 percent by weight.

In heating elements having an insulating material which consists exclusively of aluminium oxide less satisfactory results are obtained, particularly at comparatively high temperatures than with heating elements containing magnesium oxide as an insulating material and this difference is found to be eliminated by addition of boron nitride to these insulating materials.

Likewise the elements insulated with pure magnesium oxide (specific resistance p 0.5 to 1 X 10" ohm.cm at 1,000 C) yield better results as regards electrical insulation than those elements which are insulated with aluminium oxide (p 1 to 2 X 10 ohm.cm at 1,000 c On the other hand the addition of boron nitride makes it possible to eliminate this difference; at 1,000 C the mixture MgO 20 percent by weight of BN in the examined elements resulted in a specific resistance of 1 X 5 X 10" ohm.cm, while the mixtures containing A1 10 to 30 percent by weight of BN had specific resistances of from 1 X to 3 X ohm.cm, respectively.

It is known that heating elements manufactured with aluminium oxide or magnesium oxide must be protected from air humidity if their satisfactory electrical insulation is to be maintained.

Some hours of contact with the atmosphere are sufficient to seriously spoil the insulation. This phenomenon occurs to a much lesser degree in heating elements insulated with mixtures of MgO or A1 0 and EN; the heating elements according to the invention are therefore easier to handle. The values of the insulation of four heating elements may serve as Examples, which elements contain: magnesium oxide, aluminium oxide, a mixture of magnesium oxide and percent by weight of boron nitride, and a mixture of aluminium oxide and 20 percent by weight of boron nitride, respectively, as insulating materials.

The four examined samples had insulation resistances of between 10" and 10 ohm at the instant when they were contacted with air.

After 24 hours these resistances had decreased to 6 X 10" ohm, 3 X10 ohm, 2 2 X l0 ohm and l -5 X 10" ohm, respectively, for the four samples. After hours under the same circumstances values of 2 X 10 ohm, 10"ohm, 7 -2 X 10 ohm and 5 '5 X 10 ohm were found for these samples.

Due to their satisfactory properties the heating elements according to the invention may be used both for supplying high powers over a short period and for passing repeated thermal cycli (record press). A further use is the simulation of nuclear fissure elements getting hot in reactors during experimentation with these elements.

What is claimed is:

l. A heating element having a high heating current, comrpising:

A. at least one wire-like heating resistor;

B. an envelope substantially surrounding said resistor;

C. insulating material disposed between said wirelike resistor and said envelope consisting of an insulating mixture of magnesium oxide and more than approximately 20 percent by weight of boron nitride.

2. A heating element having a high heating current,

comprising:

A. at least one wire-like heating resistor;

B. an envelope substantially surrounding said resistor;

C. insulating material disposed between said wirelike resistor and said envelope consisting of an insulating mixture of aluminum oxide and more than approximately 10 percent by weight of boron nitride.

57W UNITED STATES PATENT OFFICE CERTIFICATE CORRECTION Patent Nb, 3751859 7' Dated September 25, 19 3 Inventor(s) JEAN BALDY and PATRICK COVILIaE It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On Title age insert -[30] Foreign Application Priority Data uly 31, 1970 France. .70283l8- Signed and sealed this 19th day of February 197k.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer c; MARSHALL DANN Commissioner of Patents $32253? UNITED STATES PATENT OFFICE CERTIFICATE OF CORREC Patent No. 3761859 Dated eptember 7 Inventor(s) JEAN BALDY and PATRICK COVILBE it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

n Title Page insert -[30] Foreign Application Priority Data 31 a q 0 0 c a Signed and sealed this 19th day of February l97L|..

(SEAL) Attest:

EDWARD M.FLETGHER,JR. MARSHALL N Attesting Officer Commissioner of Patents 

2. A heating element having a high heating current, comprising: A. at least one wire-like heating resistor; B. an envelope substantially surrounding said resistor; C. insulating material disposed between said wire-like resistor and said envelope consisting of an insulating mixture of aluminum oxide and more than approximately 10 percent by weight of boron nitride. 